The present invention relates to a method of treating and coating an article of cookware formed at least in part of aluminum or aluminum alloy, and to an article so coated.
Traditional non-stick cookware in particular is often made of aluminum or aluminum alloy. Although cheap and easy to form into the desired shape the material is soft, porous and stains easily. Conventionally, the aluminum surface is covered with the combination of a non-stick coating on the cooking surface and a porcelain enamel or silicon-polyester paint on the exterior. Besides providing a decorative appearance and non-stick convenience, these coverings prevent the food from interacting with the aluminum surface, which can change the taste and colour of food. Since the non-stick interior is soft and will wear out, and the pan has a limited life.
In the case of aluminum cookware, since aluminum is such a soft material it is known and increasingly popular to subject the surface of such an article to an anodizing or hard-anodizing process. In such a process, the aluminum surface functions as an anode in an electrolytic bath so that the surface becomes exposed to and reacts with the very active oxygen which is released at the anode to form an aluminum oxide layer. Hard-anodizing requires the bath temperature to be lowered. Such an anodizing or hard-anodizing process results in a much harder surface which is less susceptible to scratching or to other damage. The applicant""s EP 0424072A describes a technique for hard-anodizing, and the contents thereof are incorporated herein by reference.
Although the hard-anodizing technique greatly hardens the surface such that it becomes almost twice as hard as stainless steel, and this prevents interaction of the aluminum with foods, a problem is that the hard-anodized surface is very coarse and hard, such that it can easily abrade other surfaces such as worktops, stovetops, shelves, kitchen utensils etc which it comes into contact with and the surface quickly appears marked or marred. In addition, foods can penetrate into the hard porous surface easily staining it.
A further problem is that the hard-anodized surface cannot safely be subjected to washing in a dishwasher as the high water temperature and harsh alkaline detergents adversely affect the hard-anodized surface.
Although it would be desirable to be able to simply coat the hard-anodized surface with enamel, adherence with the porous aluminum oxide surface is very poor.
The present invention seeks to overcome these drawbacks.
According to a first aspect, the invention resides in a method of surface treating a cookware article formed of aluminum or aluminum alloy, comprising the steps of: a) applying a first coating of porcelain enamel to the exterior of the article; b) subjecting the interior of the article to hard-anodizing; and c) applying a second coating of porcelain enamel over the first coating.
The method results in an article of cookware which has a high quality, high gloss porcelain enamel-coated outer surface, and a hard-anodized interior which can if desired coated with a non-stick material.
The application of the porcelain enamel in two steps is found to be necessary since although not immediately apparent, the invention has found through testing that the anodizing acid attacks and corrodes the porcelain surface during hard-anodizing. When the pan is subjected to application of a non-stick coating and specifically the baking procedure to the non-stick coating, the porcelain surface exhibits greatly reduced the surface gloss. This problem is overcome by the two-step application procedure.
The porcelain enamel may be applied as a porcelain slip which is cured at an elevated temperature, preferably at a temperature which is sufficient to at least partially remelt the surface of the first porcelain enamel coating. This results in an enamel coating in which the second enamel coating is intimately bonded to the first enamel coating to form one single composite coat.
The first porcelain enamel coating is applied as a layer of thickness in the range 25 to 35 microns, whilst the second porcelain enamel coating is applied as a layer of thickness in the range 30 to 35 microns. The curing temperature of first and second porcelain coatings is in the region of 540xc2x0 C. to 555xc2x0 C., and the time of curing in the range 1 to 1xc2xd minutes.
The anodizing time is preferably minimised, to about 20 minutes or less.
In a further aspect the invention resides in a method of forming an article of cookware of aluminum or aluminum alloy, comprising the steps of: i) providing a disc-like blank of flat metal; ii) forming the article by stamping into the desired shape; iii) applying a first coating of porcelain slip to the exterior of the article of thickness in the range 25 to 35 microns and curing at an elevated temperature to produce a hard enamel; iv) subjecting the interior surface to hard-anodizing; v) applying a second coating of porcelain slip of thickness in the range of 30 to 35 microns over the first coating and curing to produce a hard enamel; and vi) applying a non-stick coating to the hard-anodized interior surface of the article.
The invention also resides in an article of cookware when formed according to the methods described above.
In a still further aspect the invention resides in an article of cookware of aluminum or aluminum alloy having an exterior coating of porcelain enamel, and an interior hard-anodized surface covered in a non-stick coating. The total thickness of the porcelain enamel is in the range 60 to 70 microns.
An article of cookware so coated can be safely placed in a dishwasher without risk of damage.